Wearing a brown, mop top wig, blue striped turtleneck and tight, white skinny jeans, Fallon looked every inch of an awkward '70s dancer when he came onstage last night, setting the scene for one of the funniest performances we've seen yet (and there have been a lot!)

"Everybody's looking at my tight pants," Fallon sings. "I got my tight pants, I got my tight pants on." (We're telling you: this tune will be stuck in your head for the rest of the day.)

But the comedian's happy-go-lucky mood is soon destroyed once the 'First Love' singer takes the stage, dressed in an eerily similar outfit, including (gasp!) matching tight, white pants that she sings about with fervor.

As soon as Fallon takes issue with J. Lo's matching ensemble, Lopez sticks up for her white pants like there is no tomorrow.

"Well, that's funny, because yesterday I ran into Mayor Barleycorn and he presented me with a plaque that declared my pants the tightest in all the land," she tells Fallon, who quickly contests her argument.

And that's when Lopez gets serious.

"Listen up you little b----," she demands, pointing her finger in Fallon's face. "You better hide your wife, you better hide your kids, because I will cut you. I will cut your father, I will cut your mother. I will scratch you. Don't make me take off my heels!"

C'mon, Jimmy. You know better than to mess with Jenny From the Block -- especially when she's wearing her tight pants!